The present invention relates to speech packet communication systems for communicating encoded speech signals in the form of a packet, and more particularly to such systems in which a plurality of terminals are interconnected via a communication network in which each terminal prepares packets from encoded speech signals and communicates with each other using the packets.
Since, generally, correlation between adjacent samples is high in a speech signal, it is recommendable to linearly predict an input speech signal, to subtract the predicted value from the input signal, and to quantize differentials from which correlation between samples is greatly reduced rather than to directly quantize the input speech signal, because the former provides substantially the same speech quality using a less number of quantization bits than the latter. An encoding system employing this principle is referred to as Differential Pulse Code Modulation (hereinafter referred to as DPCM briefly).
Since correlation between adjacent samples of a speech signal changes with phoneme, it is necessary to adapt a prediction coefficient, used in linear prediction, to the input speech. Generally, a predicted error or the magnitude of the differential is observed and the predicted coefficient is adapted such that the error is reduced. In the decoding end, the differential code or signal output from the transmitter is inverse-quantized or dequantized with a predetermined accuracy and a predicted value is calculated using the result and the output speech signal is obtained using the predicted value. Therefore, both the transmitter and the receiver are able to carry out encoding and decoding on the basis of the same criterion. In that case, even if the predicted coefficient itself is not transmitted actually, both the transmitter and the receiver can determine the same prediction coefficient to thereby use the transmission capacity effectively. This system is known as Backward Adaptive Differential Pulse Code Modulation (hereinafter referred to as ADPCM-b).
In a transit node in a communication network (for example, Asynchronous Transfer Mode (hereinafter referred to as ATM sorely)), certain packets have to be discarded in accordance with the traffic state. Discarding of a packet is determined in accordance with a priority assigned to the packet. Such an operation of a transit node is hereinafter referred to as congestion controlling. In view of such, a first packet is prepared from the most significant differential code bits, which greatly influences the speech quality, of a train of bits of a speech signal inputted and encoded for a predetermined interval of time and the first packet is assigned high priority, while a second packet is prepared from the least significant bits less influencing the speech quality, the second packet being assigned less priority, and the resulting first and second packets are transmitted. Assigning such predetermined priorities to the respective packets serves to provide at least the most significant bits with high probability in the receiver even if a high traffic state occurs in the transit node. If a prediction coefficient used in the ACPCM-b is calculated using only the most significant bits in both the transmitter and receiver, both the transmitter and the receiver will obtain the same prediction coefficient. Such encoding system is known as Embedded Adaptive Differential Pulse Code Modulation (hereinafter referred to as Embedded ADPCM briefly).
The ADPCM is described, for example, in AT & T Technical Journal, Vol. 65, No. 5 (1986-9, 10 pp. 12-22). The Embedded ADPCM is described, for example, in IEEE Transactions On Communications, Vol. COM-28, No. 7 (July, 1980) pp. 1040-1046, "Embedded DPCM for Variable Bit Rate Transmission" or "Variable Rate Embedded ADPCM with Perceptually Appropriate Criteria", the Institute of Electronic Information and Communication Engineers Autumn National Meeting A-4, 1988 or "Overview of CCITT Embedded ADPCM Algorithms", pp. 1014-1018, April 1990.
"Variable Rate Embedded ADPCM with Perceptually Appropriate Criteria" discloses a speech coding system of controlling the coding of a speech signal depending on the nature or property of the input speech signal, comprising the steps of calculating a power value and a prediction gain value of a speech signal input at a predetermined period and changing the number of coding bits with which the input speech signal is efficiently encoded.